The invention relates to apparatus for selectively trapping substances in a gas stream, and to systems incorporating selective traps, particularly systems for detecting nitrogen-containing compounds such as nitroso compounds, amines, nitrofurans, nitrotoluenes, and nitrogen oxides present in liquid or gaseous samples.
Among the nitrogen-containing compounds whose detection has become of increasing interest are N-nitroso compounds or N-nitrosamines, which have the general formula ##STR1## wherein R.sub.1 and R.sub.2 are the same or different organic radicals including those radicals which together with the non-nitroso N of the depicted N--NO bond constitute a nitrogen heterocyclic radical. Many of these compounds are highly carcinogenic with respect to animals. Although their effect on humans is current unknown, efforts to monitor the presence of N-nitroso compounds have increased, particularly since these compounds have been found in trace quantities in many materials which are contacted or taken internally by humans, including artificial food additives, tobacco smoke, beer, pesticides, and the atmosphere.
Considerable work has been conducted on detection and measurement of N-nitroso compounds. One system which has been successfully used to measure the N-nitrosamine content of a sample is described and claimed in the above-referenced U.S. Pat. No. 3,996,002. The system described therein includes (a) a liquid or gas chromotograph for separating in time the N-nitroso compound portion of a sample from portions containing compounds such as nitrates or nitrites; (b) a non-catalytic pyrolyzer for selectively breaking the N--NO bond in the N-nitroso compounds to selectively liberate nitric oxide (NO) gas; and (c) a NO detector for measuring the amount of NO gas liberated.
In some embodiments of the above-noted system, a cold trap is included between the pyrolyzer and NO detector for eliminating from the effluent of the pyrolyzer certain trace contaminants, carrier gases, and the pyrolysis products associated with certain solvents. A typical cold trap used in N-nitrosamine detection comprises a conduit immersed in a bath of a cryogenic medium and having an inlet and outlet for throughflow of pyrolyzer effluent. The cryogenic medium-e.g., mixtures such as dry ice and acetone, or liquid nitrogen and pentane, or liquid nitrogen and isooctane-is selected to maintain a temperature sufficiently low to condense gases in the pyrolyzer effluent whose presence in the NO detector could interfere with accurate detection of NO. This temperature must, however, be sufficiently above the freezing point of NO (about - 162.degree. C.) to permit all of the NO gas to pass through the cold trap and into the NO detector. The condensation of potentially interfering gases and retention of their condensate in the cold trap prevents their erroneous contribution to the N-nitroso compound content signal of the detector. This trapping thus helps to provide accurate detection of N-nitroso compounds and also permits the use of solvent programming and/or the use a greater variety of carrier gases in the system.
While the foregoing patented system has proven very effective in measuring the concentration of N-nitrosamines and other nitrogen-containing compounds in liquid or gaseous samples, the cold traps used as part of the system present certain drawbacks. For example, the traps use cryogenic agents which are relatively expensive and require careful handling. The cold traps also require considerable upkeep such as the periodic removal of condensate and the addition of cryogenic agent to maintain desired cold trap temperatures. The cold traps inherently do not remove gases with freezing points lower than the temperature maintained in the traps. Also, they may not remove all of the gases with higher freezing points because during flow of the gas through the trap there may be insufficient contact between some of the gas and the chilled environment of the trap. In addition, if particulate matter such as dust or dirt is present in the pyrolyzer effluent, it may pass through a cold trap, decreasing detection sensitivity and necessitating frequent cleaning of optical components of the NO detector. Moreover, the cold traps do not readily provide an indication when their performance as trapping devices deteriorates.
Accordingly, it is an object of the invention to provide an improved trap for selectively removing substances from a gas stream.
It is also an object of the invention to provide a system for measuring the content of certain nitrogen-containing compounds of a sample.
It is an object of the invention to provide a trap which, in an apparatus which determines nitrogen compound content by selective liberation in a reactor of NO gas from a sample followed by measurement of the NO gas liberated, an improved trap for removing from the reactor effluent certain substances which could interfere with accurate measurement of the NO gas liberated.
It is a particular object of the invention to provide an improved system for detecting N-nitroso compounds and having a trap according to the preceding objects which is also inexpensive, convenient to use, and requires little or no maintenance.
It is also an object of the invention to provide an improved system for detecting oxides of nitrogen in a gas stream.